1. Field of the Invention
The present invention relates to a motor drive circuit.
2. Description of the Related Art
Electronic devices such as a notebook personal computer employ a fan motor for cooling heat-generating components such as a processor. When driving the fan motor, a motor drive circuit configured to perform so-called soft-switching may be used which changes, in a gradual manner, a drive current of a motor coil to reduce the noise in the fan motor (see, e.g., Japanese Patent Laid-Open Publication No. 2004-153921).
For example, in the motor drive circuit of Japanese Patent Laid-Open Publication No. 2004-153921, a soft-switching period for changing the drive current of the motor coil in a gradual manner is constant. Therefore, for example, even when a rotational speed of the fan motor changes, the soft-switching period does not change. In such a motor drive circuit, for example, when the rotational speed of the fan motor is increased, the proportion of the soft-switching period in the period during which the fan motor is driven becomes large. Therefore, it occasionally becomes difficult to set the fan motor at a desired rotation. On the other hand, when the rotational speed of the fan motor is decreased, for example, the proportion of the soft-switching period in the period during which the fan motor is driven becomes small. Therefore, it occasionally becomes difficult to change the drive current in a gradual manner, resulting in increased noise of the fan motor.